The roundels in both cases look odd. The frame number roundels are rotated out of alignment with the number, and not on the same baseline. Although the digits are nicely square and equal spaced, they look much sharper than the roundels, like they are newer.

The engine number roundels bounce above on the left and below on the right. The right stamp looks like it is made up of multiple stampings that aren't exactly centered, and it's bigger than the roundel on the left. Again, the numbers are deep and sharp but the roundels are not. Also, I'd like to understand more about the history of the motor, that it doesn't have an OZ-80 stamp directly above the number (which would have been applied if the bike was bought or confiscated by the German military).

Strange. The Frame numbers are or the same style as my 1938 R61 and my 1939 R51 but the engine number is a different style from both my engines. I would be surprised if the engine number had not been re stamped . The stamping on my engines are both the same style and size but of a different size and style to my frame numbers. My engine numbers are vastly different to the Bonhams engine number.
Why I say strange is that as far as I am aware the major difference between pre and post war frames was post war had a support from under the seat to the bottom frame ( Which this one has ). It could have been a later addition.
The tank is in my estimation not pre war BMW . I would guess it is 1960 ies R26/R27 with the tool box cut out.
OK what is it ? I would say possibly R61/R51or R66 frame but probably restampted R51/3.
Engine probably re stamped R51 or R51/2.
Wrong tank . ( Prewar tanks are like hens teeth.)
Over all I would not touch it unless I could get it for about 500 USD and use for spares.
I am not saying I am correct without a closer examination but this is my thoughts.
I am surprised that a reputable auctioneer like Bonhams would get involved in a suspicious bike like this.
Bill
South Africa

I mostly agree with Bill's estimation, but would make the point that BMW started adding the support strut under the seat before the war (but after 1938, the year claimed for this "bike"). The tank is not correct for the model, and the motor number does look restamped to me.

Which is kind of a shame, because a project like this has enough to it that I think it's worth a lot more than $500. As a point of reference, before my participation on the 2014 Motorcycle Cannonball (where I rode my R52 across the US), I tried to buy a complete driveline that was on German eBay. I saw this as a way to have a complete set of spares for my run. I did my best to snipe* it, and bid $10k for it at the last moment, but was outbid. I suspect that I would have had to go significantly higher to get it.

A relatively complete driveline like this with a frame an forks is sufficient to compliment with lots of reproduction parts to build a bike. A restored R66 could easily be worth $30k or more, so spending $10k (the low end estimate from Bonhams) on the base is not unreasonable. There's a lot of "headroom", especially for someone who can do a lot of the work themselves.

*) On eBay, the term sniping means attempting to outbid the other buyers at the last moment of the auction so they have no time to respond. But because eBay automatically bids for you and only bids up high enough - up to your maximum bid - to win, it also means you have no time to respond if the existing high bid has more room to run.

My 1939 R51 has a down tube under the seat which I thought , prior to reading Darryl 's comments , was put in later . Now I don't know.
Only the bottom half has a bit of a flat on the mudguard side . There are some hammer marks on this flat which must have been part of some repair on the bike.
I would be interested if someone has a photo showing this flat on a unmolested frame.
I will leave my frame as is. Even if not original it is part of the bikes history.
Bill

FRame looks original r66 seria 2,engine numbers restamp,shoud looks like r61 numbers etc...
After the war got left many engine, case and heads cylinder,if is no number in it,could be old military stock.Mark Hugget told me.
This r66 case you can see its been removed original numbers.
Nice project very easy to finish it.

If this motor or frame has had numbers changed, then someone had to find a 1938-41 frame and an R61 or R71 motor and then R66 cylinders, heads and carburetors, then find out from whatever source the numbers specific to a particular R66, then restamp as needed either the frame or the motor or both, then remove all the other parts from the motorcycle not still present on these pictures, then let the remains get the patina they have? Do folks who part out very rare prewar civilian BMW twin motorcycles usually go through all that bother to make a fake bike from what's left? I'd love to have that bike, even if it's fake.

I found references to two frame upgrades for R5 and R51 in the BMW Archives. Assembly Instruction 30R, October 24, 1939, added the gussets at the head tube to both R5 and R51. Assembly Instruction 47R, September 14, 1940, added a horizontal reinforcing tube to both and the seat stay to the R51.

My understanding is that these Assembly Instructions applied to the R6, R61, R71 and R66 as well. Given that the dates of these Assembly Instructions are post production for R5/6 and relatively late in the R51/71 production, these were mostly retrofits made by dealers, so whether or not any given bike has the mods is not necessarily an indication that is was made after the dates of the Assembly Instructions. Bill's frame was likely retrofitted.

It stands to reason that similar modifications may have been made by individuals using available materials instead of BMW parts.

When I look at a plunger frame with no horizontal tube and no seat stay, I am pretty sure it is not Russian.

Ah. Thanks Bruce, You put another piece of the puzzle in place.
My bike has all these mods but they don't look like factory work. Obviously retrofitted as you say.
I enjoy the history of these bikes as much as I enjoy restoring them.

As an aside my R52 has a tiny but perfect star of David stamped on top of the gearbox . You need a magnifying glass to make it out. Would love to know the history of this but probably lost in the mists of time.
Bill
South Africa

Ok, lets start with the frame and motor numbers, Chris is 100% correct, wrong Font , fake numbers, No way in hell those are original numbers stamped into the motor and frame.

Now on to the down tube in the back, lets call it the D style. The 1939 BMW R51 I have here in my shop is no question a professional job and I feel it was done at the factory or a very good welder, its perfect. I was told that the bikes in 1939 not all but some had this. I have seen a 1939 R71 as well with this upgrade.

I can also attest to the face that my other friends BMW R51 a 39 as well DOESNT have this down D style tube.